Wavelet analysis on the turbulent flow structure of a T-junction

Experimental measurements in a T-junction with one inlet and two outlets mimicking the airflow in a high-speed train ventilation system were carried out using well-resolved Hot-Wire Anemometry (HWA). Continuous wavelet transform (CWT) is used to analyze the time-frequency contents of the instantaneous streamwise velocity; and the discrete wavelet transform (DWT) is employed to determine the multi-resolution energy characteristics. The measurements and analysis are carried out at three representative streamwise locations, i.e., upstream, mid center, and downstream of the T-junction. The results show that the normalized time-average velocity at the mid-center of the T-junction is the largest near the wall region. Comparing the CWT data in the region near the wall, it is found that the dominant frequency of the periodic high energy coherent structures increases along the streamwise direction, and the wavelet energy magnitude at mid-center of T-junction decreases with the increase of velocity ratio. The DWT results show that apparent wavelet energy peak appears at the upstream and downstream of the T-junction for different scales from 26 to 210, but not at the mid-center. However, the energy at scale 211 abruptly rises in all flow regions at all the three streamwise locations and this energy decreases with the increase of the velocity ratio. Therefore, a higher velocity ratio is preferred for suppressing the generation of large-scale coherent structures to reduce drag forces and skin frictions for high-speed trains.